Exciting premium fuel mileage

[cookie]

Before Xmas a few of us were discussing the effects of premium fuel on mileage. This is my third tank of premium so I thought I would check the mileage difference.

The results are not totally fair since I changed the front oxegen sensor just before this fill up.

Before on winter No CA regular (89) I got 23.90 mpg. This is a drop of a mpg or two from summer fuel.

Now armed with premium (91) I got.....are you ready....23.73 mpg, a drop of .17.

This is close enough to be almost identical and the fact that it has been a bit cold and rainy can easily make up the difference.

The computer was reset a couple of times since I added premium so its not a matter of relearning for the new fuel. The power is just about the same and you would need a dyno to see if there is any difference. The piston slap is about the same too. This is a 99 forester in mixed city and freeway driving.

[clutchcargo]

So are you saying that premium fuel makes no difference except to your wallet?

 

Before Xmas a few of us were discussing the effects of premium fuel on mileage. This is my third tank of premium so I thought I would check the mileage difference.

The results are not totally fair since I changed the front oxegen sensor just before this fill up.

Before on winter No CA regular (89) I got 23.90 mpg. This is a drop of a mpg or two from summer fuel.

Now armed with premium (91) I got.....are you ready....23.73 mpg, a drop of .17.

This is close enough to be almost identical and the fact that it has been a bit cold and rainy can easily make up the difference.

The computer was reset a couple of times since I added premium so its not a matter of relearning for the new fuel. The power is just about the same and you would need a dyno to see if there is any difference. The piston slap is about the same too. This is a 99 forester in mixed city and freeway driving.

[cookie]

So far on my car that would be correct. There is not a lot of difference between 91 and 89 which is what I get around here. I really didn't expect much noticeable power increase but I did think the mileage might be a mpg or two better.

[OB99W]

So are you saying that premium fuel makes no difference except to your wallet?

As long as an engine doesn't require fuel above a certain octane rating, that would be the expected outcome. Before engine management systems, you'd either give your car what it wanted, live with the knocking if you didn't (and possibly damage things), or manually retard the ignition timing (read:"turn the distributor a few degrees"). Now we have things like computers and knock sensors, and the system retards the timing for us if we try to "cheat". If a lower-grade gas causes the timing to get retarded, mileage and power are going to take a hit, and going up a grade or two helps. However, if the engine is already happy with the lower grade, timing won't get retarded, and a higher grade can't improve that. In fact, since a higher-octane gas is more resistant to burning (by definition), it may actually be a worse performer when the engine is cold.

[cookie]

That would be my conclusion so far. I think with my very high compression BMW the results might be different. This is also sea level here and folks who have reported better mileage with higher octane have been in the Rockies.

I am going to give it another tank of premium to be sure and then drop down to 87 where I used to run it. I think this car is probably designed for 87.

[SubaruLegacy2003]

I took these notes about 6 years ago in a 1990 Accord after a similar discussion. I drove pretty much the same route and similar traffic except for a few tanks where the mpg dropped considerably due to traffic:

 

 

9/22/99 360/ 13.9 25.9 87 octane

 

9/26/99 359/14.2 25.7 87 octane

 

10/4/99 366/13.7 26.7 87 octane

 

10/11/99 307/12.8 23.4 87 octane

 

10/13/99 396/14.4 27.5 93 octane

 

10/16/99 382/ 14.6 26.0 89 octane

 

10/19/99 253/ 9.8 25.8 89 octane

 

10/23/99 335/13.7 24.5 89 octane

 

10/29/99 350/15.2 23.0 89 Octane

 

11/9/99 372/14.1 26.4 87 Octane

 

I got the highest MPG with the 93, but overall it did not to seem to make much difference...

[cookie]

Conditions are not always the same so the longer one averages the more accurate the test. Since I have no patience for long tests this will be a short one. Your observations are interesting since you did get a bit more ob the 93, but perhaps the Honda could use it effectively.

[PAezb]

http://www.state.mn.us/mn/externalDocs/Commerce/Gasoline_Octane_Facts_102902052227_OctaneFacts.pdf

[cookie]

Good explanation which was pretty much as I understand that this works. The idea came up when someone said they got better mileage with high octane. I figured it was worth a try as mileage is always a topic with Subarus.

[blitz]

In fact, since a higher-octane gas is more resistant to burning (by definition), it may actually be a worse performer when the engine is cold.

I've seen a lot of disparity on this as well as the subject of the "burn rate" of premium being slower than regular. I'm no fuel expert or chemist, but I'll share what little I've learned.

 

Apparently it depends on how the octane is achieved. If it's done by addition of metallic salts (TEL, MMT, etc.) the ignitability and burn rate are interfered with somewhat.

 

US premium currently uses neither, but instead blends more aromatics (tolene?). As far as I've been able to find out, it doesn't burn any slower or faster, but does resist self-ignition and does vaporize well so can aid cold-starting.

 

I think Canada still uses MMT to build octane. I'm not sure about any other countries.

[1 Lucky Texan]

I took these notes about 6 years ago in a 1990 Accord after a similar discussion. I drove pretty much the same route and similar traffic except for a few tanks where the mpg dropped considerably due to traffic:

 

 

9/22/99 360/ 13.9 25.9 87 octane

 

9/26/99 359/14.2 25.7 87 octane

 

10/4/99 366/13.7 26.7 87 octane

 

10/11/99 307/12.8 23.4 87 octane

 

10/13/99 396/14.4 27.5 93 octane

 

10/16/99 382/ 14.6 26.0 89 octane

 

10/19/99 253/ 9.8 25.8 89 octane

 

10/23/99 335/13.7 24.5 89 octane

 

10/29/99 350/15.2 23.0 89 Octane

 

11/9/99 372/14.1 26.4 87 Octane

 

I got the highest MPG with the 93, but overall it did not to seem to make much difference...

 

If a '90 model Honda had a knock sensor (anyone?) the above tests should've had the ECU reset before switching to the higher octane.

 

I dunno

[D-Cal]

This is an excellent (but long) read on MMT:

http://healthandcleanair.org/mmt/icct.pdf

Canada banned MMT in a roundabout way around 97 and was sued by Ethyl Corp. under the provisions of NAFTA for damages and it was eventually allowed again. 95% of refiners are voluntarily not using it, so it is not widely used in Canada but you have to check where you fill up.

 

The US EPA was also sued by Ethyl and MMT was federally allowed again in the US starting in 2004, except where banned by the state (CA). Some small refiners use it, again, check where you fill up.

 

If your plugs or combustion chamber have a reddish tinge to them, your regular fill up refiner is using MMT. If you've ever used an octane booster while tuning a high-pressure turbo engine, quite often they leave the same deposits. It can foul your O2 sensor(s) quite badly.

 

Toluene and xylene are quite often used as octane boosters as well, but both are highly carcinogenic. Not great alternatives.

 

Most gas stations in Canada that I have personally observed are using ethanol up to 10%. Husky/Mohawk fuels is building a massive plant near their refinery on the Alberta/Saskatchewan border to produce it from corn and other local produce.

 

With regards to gas mileage (the real topic of this thread, lol), you will obtain the best mileage by supplying the engine with the octane it is tuned to require. Try advancing the base ignition timing a few points and try again with the premium, you will probably notice a small increase in low end power. Also, to run the test accurately you should datalog for knock to know how the ECM is regarding your fuel.

[OB99W]

In fact, since a higher-octane gas is more resistant to burning (by definition), it may actually be a worse performer when the engine is cold.

I've seen a lot of disparity on this as well as the subject of the "burn rate" of premium being slower than regular. I'm no fuel expert or chemist, but I'll share what little I've learned.

 

Apparently it depends on how the octane is achieved. If it's done by addition of metallic salts (TEL, MMT, etc.) the ignitability and burn rate are interfered with somewhat.

 

US premium currently uses neither, but instead blends more aromatics (tolene?). As far as I've been able to find out, it doesn't burn any slower or faster, but does resist self-ignition and does vaporize well so can aid cold-starting.

Sheesh, I just can't seem to get away with using imprecise/less-technical language such as "resistant to burning" . Okay, what I really meant was "higher-octane gasoline burns with a more-controlled flame front, exhibiting less tendency for compression from the advancing pressure-wave to cause spontaneous ignition of the remaining vapor, as compared to lower-octane gas". Better ?

 

Yes, results with premium vary, depending on formulation, which is why I said "it may actually be a worse performer when the engine is cold" (emphasis added). Although it varies with area and season, on average premium is slightly less volatile than regular. Additional toluene and other aromatics are used in some areas, but in California (where Cookie is apparently located), aromatics are limited in order to meet standards for CBG (Clean Burning Gas).

 

Have a knock-free and high-mpg new year, everyone!

[blitz]

99OBW sorry if you missed my intent. I wasn't being critical of your info, only adding to it.

 

Knowing that the octane number can be had by more than one means goes a long way towards explaining why some folks swear their regular-speced engine runs "crisper" on premium when others claim just the opposite.

 

Bracket racers would use MMT-laden premium fuel to slow their cars by a tenth or so after running their dial-in on regular. Maybe they still do, I don't know.

 

Don't drink on new years everybody, a hangover is the worst way to start a new year.

[D-Cal]

Bracket racers would use MMT-laden premium fuel to slow their cars by a tenth or so after running their dial-in on regular. Maybe they still do, I don't know.

 

It's also an excellent way to check for even fuel distribution. The reddish tinge on a new set of plugs is a fairly good indicator.

[blitz]

Yeah I clearly remember that look (like rust). I gotta say that the fuel quality at the BP/Amoco I frequent is excellent. If pull the plugs at 10k miles just to have a look-see, the ceramic and electrodes both are spotless with just a tinge of tan & green. The source of the green is a mystery to me. I run premium in my regular-speced 2.5 because it knocks on regular. 93 gives me enough headroom to bump the timing by about 5 degrees and still remain below the threshold of knock in mid-summer.